Method of making and using neutral sulphite pulp



March 13, 1962 A. M. THOMSEN METHOD OF MAKING AND USING NEUTRAL SULPHITE PULP Filed April l5, 1959 tates As used today in the pulp and paper industry the use of the so-called neutral sulphite cook is virtually conned to hardwoods. Contrariwise, I find that it is of almost unlimited application, not only in the pulp industry, per se, but in general manufacturing chemistry as well. In the making of a high grade paper pulp from conifers, for instance, a better pulp can be produced than, say, by the kraft process, if a much larger amount of sulphite is used and if this be free from such harmful substances as thiosulphates. Unfortunately, the reverse is the modern application, due in no small part to the lack of a really effective and cheap method of regeneration of the cooking liquor. I believe that such a recovery system has been found by me and it is herein described. Y

But in addition to this high-grade phase there is a far more important one, namely the use of any type of cellulosic raw material as starting material for the manufacture of a pulp, consisting largely of cellulose but destined not `for the paper machine but for conversion into sugar by 'any well known type of saccharication. The uses of such sugar, at a price, opens a door into a new chemical world of the greatest importance, of almost limitless pos-` sibilities.

But apart from the tree the vegetable world furnishes us with endless quantities of a cellulose totally unfit for paper. If such material be pulped by any standard technique, save the neutral sulphite cook, a great deal of this cellulose, or perhaps better called poly-saccharides, is likewise dissolved in the cooking liquor. Two damages are thus sustained simultaneously, namely, the amount of cellulose obtained in separated form is greatly diminished and so much cooking chemical is consumed in the operation that the process perishes because the cost per pound of cellulose thus obtained is prohibitive. The fact that neutral sulphite is essentially a solvent for lignin, with but little effect on the poly-saccharides, makes it an ideal tool for such cellulose separation.

But as this type of pulp has no use in paper making the use of such pulp is economically bound up with its manufacture. I have already indicated that it saccharities readily, but such sugar is in the form of a very dilute solution 'and its use is again a mandatory addition to its manufacture. Endless and profitable ways and means are represented when this sugar is fed at once to micro-organisms that again transform the sugar into the widest scope of commercial products. A special class is the type that poisons itself by its own activities so that we nd it necessary to add large amounts of lime to form insoluble, or sparingly soluble salts with such poisons and thus remove them virtually as fast as they form. In most, if not all cases such lime salts can be converted by treatment with sulphur dioxide into calcium sulphite, which is almost insoluble, and the liberated organic compound which is the object of the process. This lreaction between said organic lime salts and sulphur dioxide is then advantageously woven into the recovery system for the spent cooking liquor making one process out of the entire operation, both technically and commercially.

A special case of such a combination has been picked by me and is illustrated in the drawing. While not particularly lange in its commercial scope it does contain almost every step in what I consider my process land said process is best understood by following the various interlocking steps as I have put them down. I have taken ice wood as my raw material, but as already indicated I mean by this expression tree refuse from which every bit of better material has been separated as lumber and as paper mill chips so its only destination is the tire box. I pulp this material with neutral sulphite of soda in conventional manner, but I add my own version of the recovery system. In saccharitication I am again conventional in the actual chemistry involved, but most unorthodox in technique or in using a specific type of paper pulp. Wood saccharication, to the extent that I am familiar therewith, operates on the crude wood, not on pulp, and particularly not on the pulp I am using herein.

As the micro-organism selected I have taken Bacterum Iacts acidi, which will not tolerate much lactic acid but functions perfectly if this be neutralized as fast Ias it forms by calcium carbonate, preferable in a precipitated form. Such calcium lactate as is formed is somewhat soluble, though harmless, hence it permits me to call attention to another step, not shown on the drawing, but at times important. Contrariwise, had I selected A. niger, then the lime would appear as the citrate and oxalate, both of which are substantially insoluble. In that event I would have lacked a re-cycle step which I intend to refer to later. In the recovery system, per se, the important step is to obtain -a solution las free as possible from thiosulphates. I obtain my goal by smelting in conventional manner, crystallizing sodium carbonate from the green liquor made from said smelt, and then smelting a second time the mother liquor with certain minerals thus removing said sulphur from the circuit.

Commencing then with a description of the drawing, I have shown at the upper left a saturator which is but an4 agitated tank where the wood is steeped in boiling hot cooking liquor prior to pressure digestion. Obviously, to be thus treated, said Wood must be suitably comminuted. It is then pumped to the digester where it is heated under a pressure of approximately 175 pounds gage, for several hours until completely pulped, the time naturally Varying with the character of the wood but two hours will probably be about a mean.

This digestion may be on a batch basis but I prefer to have the digested mass leave the digester in theform of a continuous jet, thus` enabling the liberated steam to be used elsewhere, sayin subsequent evaporation of the spentcooking liquor. Such liberation of steam takes place in what I have called the blowpit which is but a tank from which pulp and spent liquor is continuously withdrawn'` for subsequent washing. On a batch basis, of course, there will be no salvage of steam and the pulp will be washed free from spent liquor in said blowpit.

Separation of the pulp having been made, itis next converted to sugar by a second digestion in a dilute sulphuric acid solution at a temperature corresponding to pounds gage pressure for but very few minutes, generally less than 10 minutes. It is not the aim of my process to convert all the pulp at one time. Contrariwise, I stop when only one-half has been converted and then separate the residual pulp from the sugar solution. The separated pulp is then re-cycled to the saccharifying step once more. This whole operation takes place in the sacchariiier, which is but another digester, again preferably operated on a continuous basis, the mixture of pulp and sugar solution issuing in the form of a jet as inV the case of the pulp digester. On the drawing I have shown the liberated steam used in an evaporator for spent liquor as previously suggested, but not indicated, in the pulping phase. f

In the saccharifier the acidity of the pulp is made to correspond to a 1/z% of sulphuric acid and the time of residence is governed by actual performance, namely, the yield in sugar should equal the weight of pulp consumed.

There is, of course, some loss due to decompositionof the sugar formed, but such loss should be balanced by the increase in weight due to hydration, one molecule of pulp theoretically requiring two molecules of water for its conversion into sugar.

While I have shown a lactic acid fermentation in the drawing the Widest latitude prevails. As long as a calcium salt is precipitated by the addition of carbonate, and as long as said calcium salt will be decomposed by sulphur dioxide to form calcium sulphite it Will, manifestly, fulfill the requirements of my process irrespective of the actual type of fermentation involved. In all such fermentation the required nutrients for the growth of the organism is, obviously, added, but such addition is greatly diminished by the re-cycle step shown in the drawing where a part of the fermenting brot is continuously re-cycled from the last to the' first vessel in the series. Fermentation is finished when such re-cycled material is substantially sugar free.

With fermentation finished the calcium salt formed is next commingled with sulphur dioxide in the decomposer, which may be anything from a bubble tower to a simple agitated tank. Filtration then separates the calcium sulphite formed from the lactic acid solution and said sulphite is re-cycled to the recovery process for cooking liquor. When desired such a filtration step can be before the decomposer, thus giving a purer form of the organic component of the calcium salt. In that case the filtrate, containing the nutrients for the organism employed can be re-cycled as the medium for the pulp suspension that Ienters the saccharification step with consequent economy in the use of such nutrients.

Taking up now the regeneration of the cooking liquor I have indicated the spent cooking liquor as evaporated, smelted, and dissolved in the conventional manner of any kraft mill. However, in the dissolver I have indicated a solution of sodium carbonate from a later step as the liquid medium employed in place of water. In addition, it is the objective here to make a very concentrated green liquor so that simple cooling will give an abundant crystallization of sodium carbonate as the decahydrate. In the crystallizer, therefore, this takes place and a centrifuge separates said sodium carbonate from the mother liquor which is essentially a sulphide of sodium. The separated crystals of sodium carbonate form the first step in regeneration of the spent liquor. I have shown them as reacted with the calcium sulphite coming from the decomposer, thus giving the neutral sulphite of sodium, which is the active agent in the cooking liquor, as a solution, while the calcium sulphite is converted to the carbonate. This compound is, in turn, re-cycled to the fermentation step to neutralize the lactic acid produced by the organism. It may be, at times, that the production of calcium sulphite and sodium carbonate do not balance perfectly, so a deficiency of calcium sulphite is corrected for by a supplementary use of sulphur dioxide in the sulphitizer from which the regenerated cooking liquor emerges in finished form, to be re-cycled to the saturator.

I have next shown what happens to the sodium sulphide solution from which the sodium carbonate was separated. From the centrifuge it goes to a drier Where it is mixed with finely divided chrornite, i.e., a naturally occurring combination of the oxides of chromium and iron as mineral. The sodium sulphide acts as the cementing material to the mixture so that the product is obtained in granular form, entirely suited for the electric furnace which is the next step, and in which said oxides are converted to sulphides.

Inasmuch as the operation of an electric furnace is entirely conventional in many industries, though novel in a pulping plant, it does not seem to need any special description here. The fused mass from the furnace is run, continuously, into water in the same manner as the kraft smelt previously referred to and here it disintegrates and partially dissolves. The suspended solids are the sulphides of iron and chromium, the solution represents the sodium compounds, chiefly caustic soda, with some sulphide. It is separated from said solids and re-cycled to that previous dissolver that received the product from the smelten but as neither caustic soda nor sodium sulphide will crystalize under the conditions of the former dissolver said solution is first carbonated With any carbon dioxide containing gas. All soda is thus completely re-cycled, but the sulphur remains in combination with the solids.

As a supplementary step I have, therefore, indicated the reaction of these sulphides with dilute sulphuric acid to yield a solution of the respective sulphates, the form in which they are acceptable for further processing, and hydrogen sulphide gas. This gas, in turn, is then burned with air in a combustion chamber and furnishes most of the sulphur in re-cycled form. However, as losses are unavoidable I have shown a supplementary sulphur burner to make up any deficiency in sulphur as sulphur dioxide.

In the selection of the mineral to perform in this manner there is also considerable latitude. While I have shown only a mixture of iron and chromium oxides, other metals will also serve. Thus, the chromium may be replaced in any degree with titanium and/or zirconium, with even economic advantage. In the latter case, commingled With the solution of sulphates would be a basic titanium and/or zirconium compound, insoluble in said solution, hence easily separated and reserved for further treatment.

Throughout my explanations I have consistently used names that indicate sodium salts, only, but it is obvious that the corresponding potassium salts would serve as well. Obviously, I regard such a substitution as being within the limitations of my disclosure. Having thus fully described my process, I claim:

1. The method of making and using neutral sulphite pulp which comprises; saturating cellulosic material with a solution of neutral sodium sulphite and digesting under a pressure of approximately 175 pounds gage, until the resident cellulose shall have been substantially liberated from encrusting material; separating said cellulose from the spent cooking liquor; saccharifying the cellulose by digesting under a pressure of approximately pounds gage, with an approximate 1/2 of sulphuric acid in water solution until partially sacchariiied; separating the resultant sugar solution and re-cycling the separated cellulose to saccharication; neutralizing and fermenting the resultant sugar solution with an organism requiring neutralization of the products of its life functions with calcium carbonate to form sparingly soluble calcium salts; decomposing said calcium salts subsequently with sulphur dioxide to form calcium sulphite and liberating the organic compound produced during fermentation; reserving said calcium sulphite for further use; evaporating and smelting in conventional manner the spent cooking liquor; dissolving said smelt in a solution of sodium carbonate re-cycled from a subsequent step in the recovery system; crystallizing sodium carbonate out of the resultant green liquor, and separating said carbonate; commingling a solution of said sodium carbonate with the calcium sulphite previously obtained and reserved to form calcium carbonate and a solution of sodium sulphite; re-cycling said calcium carbonate to the fermentation phase as the neutralizing material and re-cycling the solution of sodium sulphite as a regenerated cooking liquor; commingling the solution, consisting largely of sodium sulphide, obtained when the crystallized sodium carbonate was removed from its mother liquor, with powdered mineral selected from the group consisting of iron, chromium, titanium, and zirconium, in oxide form, and drying the mixture; fusing said mixture in the electric furnace and commingling the resultant smelt with water; separating the sodium salts thus obtained in solution from insoluble materials; carbonating the sodium salts in said solution thus obtaining the solution for re-cycling as previously referred to in dissolving the smelt from the spent liquor smelting furnace; dissolving the insoluble material separated from said liquor in dilute acid to obtain a solution of the metal involved and an evolution of the resident sulphur as hydrogen sulphide; burning said hydrogen sulphide and recycling the sulpuhur dioxide thus produced to Where called for in the process.

2. The method of making and using neutral sulphite pulp set forth in claim 1, with the added step that the pulping step be performed on a continuous basis, the mixture of spent cooking liquor and pulp emerging con- -tinuously in the form of a jet from the pressure vessel in which said digestion takes place, and using the liberated steam in the evaporation of the spent cooking liquor.

3. The method of making and using neutral sulphite pulp set forth in claim 1, with the added step that saccharication of the pulp formed therein be on a continuous basis, the mixture of saccharifying uid and pulp being discharged continuously in the form of a jet from the confining vessel, the liberated steam being employed in evaporation of dilute solutions obtained in the process.

4. The method of making neutral sulphite pulp which comprises; saturating cellulosic material with a solution of neutral sodium sulphite and digesting under a pressure of approximately 175 pounds gage, and the corresponding temperature until the resident cellulose shall have been substantially liberated from encrusting materials; separating said cellulose from said spent cooking liquor;

evaporating said liquor and smelting in the conventional kraft manner; dissolving the resultant smelt in a re-cycled solution of sodium carbonate obtained in a later step; crystallizing the resultant green liquor and removing the crystals of sodium carbonate thus obtained; neutralizing said crystals with sulphur dioxide thus obtaining a regenerated cooking liquor; commingling the mother liquor from said crystallization with a powdered mineral selected from the group consisting of the oxides of iron, chromium, titanium and zirconium; drying the mixture and fusing in an electrically heated furnace; commingling the resultant fusion in Water to obtain a slurry of soluble and insoluble substances; separating the soluble portion, consisting essentially of sodium salts, and re-cycling same as the dissolving liquor for the smelt obtained in the former smelting of the spent cooking liquor as previously prescribed, thus recycling to the regenerated liquor the remainder of the sodium salt used in the primary digestion.

References Cited in the le of this patent Thomsen Aug, ll, 1959 

1. THE METHOD OF MAKING AN USING NEUTRAL SULPHITE PULP WHICH COMPRISES; SATURATING CELLULOSIC MATERIAL WITH A SOLUTION OF NEUTRAL SODIUM SULPHITE AND DIGESTING UNDER A PRESSURE OF APPROXIMATELY 175 POUNDS GAGE, UNTIL THE RESIDENT CELLULOSE SHALL HAVE BEEN SUBSTANTIALLY LIBERATED FROM ENCRUSTING MATERIAL; SEPARATING SAID CELLULOSE FROM THE SPENT COOKING LIQUOR; SACCHARIFYING THE CELLULOSE BY DIGESTING UNDER A PRESSURE OF APPROXIMATELY 150 POUNDS GAGE, WITH AN APPROXIMATE 1/2% OF SULPHURIC ACID IN WATER SOLUTION UNTIL PATIALLY SAACHIRIFED; SEPARATING THE RESULTANT SUGAR SOLUTION AND RE-CYCLING THE SEPARATED CELLULOSE TO SACCHARIFICATION; NEUTRALIZING AND FERMENTING THE RESULTANT SUGAR SOLTUION WITH AN ORGANISM REQUIRING NEUTRALIZATION OF THE PRODUCTS OF ITS LIFE FUNCTIONS WITH CALCIUM CARBONATE TO FORM SPARINGLY SOLUBLE CALCIUM SALTS; DECOMPOSING SAID CALCIUM SALTS SUBSEQUENTLY WITH SULPHUR DIOXIDE TO FROM CALCIUM SULPHITE AND LIBERATING THE ORGANIC COMPOUND PRODUCED DURING FERMENTATION; RESERVING SAID CALCIUM SULPHITE FOR FURTHER USE; EVAPORATING AND SMELTING IN CONVENTIONAL MANNER THE SPENT COOKING LIQUOR; DISSOLVING SAID SMELT IN A SOLUTION OF SODIUM CARBONATE RE-CYCLED FROM A SUBSEQUENT STEP IN THE RECOVERY SYSTEM; CRYSTALLIZING SODIUM CARBONATE OUT OF THE RESULTNT "GREEN LIQUOR," AND SEPARATING SAID CARBONATE; COMMINGLING A SOLUTION OF SAID SODIUM CARBONATE WITH THE CALCIUM SULPHITE PREVIOUSLY OBTAINED AND RESERVED TO FORM CALCIUM CARBONATE AND A SOLUTION OF SODIUM SULPHITE; RE-CYCLING SAID CALCIUM CARBONATE TO THE FERMENTATION PHASE AS THE NEUTRALIZING MATERIAL AND RE-CYCLING THE SOLUTION OF SODIUM SULPHITE AS A REGENERATED COOKING LIQUOR; COMMINGLING THE SOLUTION, CONSISTING LARGELY OF SODIUM SULPHITE, OBTAINED WHEN THE CRYSTALLIZED SODIUM CARBONATE WAS REMOVED FROM ITS MOTHER LIQUOR, WITH POWDERED MINERAL SELECTED FROM THE GROUP CONSISTING OF IRON, CHROMIUM, TITANIUM, AND ZIRCONIUM, IN OXIDE FROM, AND DRYING THE MIXTURE; FUSING SAID MIXTURE IN THE ELECTRIC FURNACE AND COMMINGLING THE RESULTANT SMELT WITH WATER; SEPARATING THE SODIUM SALTS THUS OBTAINED IN SOLUTION FROM INSOLUBLE MATERIALS; CARBONATING THE SODIUM SALTS IN SAID SOLUTION THUS OBTAINING THE SOLUTION FOR RE-CYCLING AS PREVIOUSLY REFERRED TO IN DISSOLVING THE SMELT FROM THE SPENT LIQUOR SMELTING FURNACE; DISSOLVING THE INSOLUBLE MATERIAL SEPARATED FROM SAID LIQUOR IN DILUTE ACID TO OBTAIN A SOLUTION OF THE METAL INVOLVED AND AN EVOLUTION OF THE RESIDENT SULPHUR AS HYDROGEN SULPHITE; BURNING SAID HYDROGEN SULPHIDE AND RECYCLING THE SULPHUR DIOXIDE THUS PRODUCED TO WHERE CALLED FOR IN THE PROCESS. 